1. Field of the Invention
The present invention generally relates to remediation of soil contaminated by volatile organic compounds, and in particular to a system for in-situ soil vacuum extraction of the contaminates and the process thereof.
2. Description of the Prior Art
Conventional methods of removing contamination from groundwater and soils include excavating the contaminated subsurface material and pumping contaminated water from subsurface materials to the surface for treatment. Both methods are expensive due to the volume of material which must be removed, treated, and disposed.
In-situ soil vacuum extraction (ISVE) involves the removal of volatile organic compounds (VOC's). Dissolved VOC's are present in the upper, unsaturated "vadose" zone above the groundwater. The dissolved VOC's approach an equilibrium concentration in the soil's pore space according to Henry's Law.
The contaminates can be aliphatic and/or aromatic hydrocarbons, halogenated hydrocarbons, or other volatile organic compounds. Many of these compounds have vapor pressures of at least 20 mmHg at room temperature. Accordingly, such compounds can be easily volatilized when subjected to a suitable vacuum or air flow.
The design and operation of ISVE remediation systems are based on the movement of gases in a porous media. Thus, a negative air pressure gradient exerted on the soil will induce migration of the subsurface VOC's. The VOC's can then be collected at extraction point(s) and discharged and collected at the surface. Accordingly, an ISVE remediation system must be designed in accordance with site specific subsurface conditions in order to maximize the rate of contaminant removal.
ISVE remediation systems may be either vertical or horizontal wells. Vertical systems become cost competitive with excavation and removal when the vadose zone exceeds about 20 feet in depth. Horizontal systems are more effective where contaminates are very shallow, i.e. less than about 20 feet. However, vertical well systems are prone to plugging.
The vacuum at the wellhead of conventional ISVE remediation systems is directly related to the range of influence (ROI) at the well and, therefore, the rate of removal of VOC's from the site. In addition, it is well known that the spacing of the wells between one another of conventional ISVE remediation systems is critical to the overall performance of the ISVE remediation system. This is due, in part, because the ROI can vary depending on the soil type and depth of groundwater.
For example, Malot and Wood applied a ISVE remediation system. at a site in which 15,000 gallons of CCl.sub.4 was spilled in an area where the top of the unconfined aquifer was 300 feet below the surface. Slotted pipes were installed at depths of 75 to 180 feet. A vacuum of 29.9 inches of Hg and a flow rate of 240 CFM was applied. After 90 days the vacuum stabilized at a ROI of 10 feet (Malot, James J. and Wood, P. R., "Low Cost, Site Specific, Total Approach to Decontamination", Conference on Environmental and Public Health Effects of Soils Contaminated with Petroleum Products, University of Massachusetts, Amherst, MA. Oct. 30-31, 1985).
U.S. Pat. Nos. 4,593,760 and Re. 33,102 (U.S. Pat. No. 4,660,639), issued to Visser et al., describe one ISVE remediation system for removing VOC's from the vadose zone. Wells are sunk vertically into the vadose zone. The well casing includes a lower perforated region. VOC's in the vadose layer enter the perforated casing and are pumped to the surface for treatment.
U.S. Pat. No. 4,832,122, issued to Corey et al., also describes another ISVE remediation system for removing VOC's from the vadose zone. Two sets of wells are sunk into the vadose zone. One well injects a fluid into the saturated zone below the plume of contamination. A second well, located above the plume, collects the fluid along with the VOC's from the plume, and pumps it to the surface for treatment.
ISVE remediation systems can have a great degree of success removing VOC constituents exhibiting relatively high vapor pressures and under the proper hydrogeologic settings. In addition, ISVE remediation systems are very cost competitive when compared with other alternatives, including physical removal and disposal in a secure landfill. However, conventional ISVE remediation systems perform poorly for areas having relatively high water tables and/or soils with an extremely high clay content.
It has thus become desirable to develop an improved ISVE remediation system for VOC contaminants which is more cost effective than a conventional ISVE remediation systems while, at the same time, eliminating the prior art problems of poor performance in areas having relatively high water tables due to high water lift and/or soils with an extremely high clay content. In addition, the improved ISVE remediation system should have a ROI at least equal or greater than a conventional ISVE remediation system and be less dependent on spacing between adjacent wells.